1. Field of the Invention
The present invention relates to methods, apparatus, and systems for obtaining information regarding a geological formation or a well passing through a geological formation. The present invention more particularly relates to methods, apparatus, and systems for exchanging information and power between an interrogating tool located in a cased borehole and sensors attached to the casing.
2. State of the Art
The extraction of oil and natural gas from a geological formation is usually accomplished by drilling boreholes through the subsurface formations in order to reach hydrocarbon-bearing zones, and then using production techniques for bringing the hydrocarbon to the surface through the drilled boreholes. To prevent the boreholes from collapsing, boreholes are often equipped with steel tubes called casings or liners which are cemented to the borehole wall. Once they are put in place, casings and liners preclude direct access to the formation, and therefore impede or prevent the measurement of important properties of the formation, such as fluid pressure and resistivity. For this reason, the logging of wellbores is routinely performed before the casing is set in place.
In order to optimize the depletion of the reservoir, it is highly desirable to monitor the temperature, pressure and other formation parameters at different depths in the well, on a permanent basis, over most of the life of the well. Valuable information regarding the integrity of the wellbore can be gained from continuously monitoring parameters such as well inclination and casing thickness. A common approach to such monitoring consists of attaching sensors to the outside of the casing, interconnecting the sensors via cables to provide telemetry and power from the formation surface, and cementing the sensors and cables in place. A description of such a system is provided in U.S. Pat. No. 6,378,610 to Rayssiguier et al. Such a system has numerous apparent drawbacks such as complicating the installation of the casing and the impossibility of replacing failed components. Another monitoring system is disclosed in U.S. patent application Ser. No. 2001/0035288 to Brockman et al. which discloses means for exchanging information and power through the casing wall via inductive couplers. These couplers, however, require extensive modification of the casing and are not suitable for an installation in situ. In previously incorporated U.S. Pat. No. 6,070,662 Ciglenec et al., means are disclosed for communicating with a sensor implanted in the formation, but this arrangement requires that the sensor be put in place prior to the installation of the casing. U.S. Pat. No. 6,443,228 to Aronstam et al. describes means of exchanging information and power between devices in the borehole fluid and devices implanted in the wellbore wall, but does not consider the problems introduced by the presence of a casing or a liner.